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  • H01L23/36—Selection of materials, or shaping, to facilitate cooling or heating, e.g. heatsinks
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Definitions

• the present invention relates to a copper-ceramic composite and a module containing this composite, which can be used in power electronics components. Ceramic circuit carriers are high due to their high thermal conductivity
• DCB-method Direct-Copper-Bonding
• AMB-method Active-Metal-Brazing
• the composite material obtained after metallization of the ceramic substrate is also referred to as metal-ceramic substrate or metal-ceramic composite.
• metal-ceramic substrate or metal-ceramic composite.
• DCB substrate is often used.
• oxygen is used to reduce the melting point of the copper from 1083 ° C to the eutectic melting temperature of 1065 ° C.
• the oxidation of copper foils before the metallization of the ceramic substrate or by supplying oxygen during the high-temperature process creates a thin eutectic melt layer. This reacts with the surface of the ceramic substrate, so that ceramic and metal can be firmly bonded together.
• DCB processes are described, for example, in US Pat. No. 3,744,120 or DE 23 19 854.
• the metallization can, for example, only on one side of the ceramic substrate ("single-layer bonding" SLB) or alternatively on both sides of the
• the metal-ceramic composite is subject to high thermal cycling, where significant
• Temperature changes (e.g., in the range of -40 ° C to + 150 ° C) may occur.
• DE 40 04 844 A1 and DE43 18 241 Al describe metal coatings on ceramic substrates, which have at their edges edge attenuations in the form of depressions or holes.
• thermal cycling occurring during operation can lead to cracks in the ceramic.
• mechanical stresses can occur during transport or installation in an electronic component, which also cause cracks in the ceramic.
• the ceramic substrate of the metal-ceramic composite should have the highest possible mechanical strength (for example in the form of a high bending strength).
• the ceramic substrate of the metal-ceramic composite is defined in more detail with regard to its grain structure (ie its structure at the microscopic level).
• the ceramic substrate contains a zirconia-reinforced alumina wherein the average grain size of the alumina ranges from 2-8 ⁇ is and the ratio of the length of the grain boundaries of Al 2 0 3 grains to the total length of all grain boundaries> 0.6.
• this grain structure contributes to the improvement of the thermal conductivity.
• An object of the present invention is to provide a metal-ceramic composite having an improved property profile, in particular a high mechanical strength.
• the ceramic substrate of a copper-ceramic composite is generally a polycrystalline material consisting of small crystallites (also referred to as grains). On a microscopic level, polycrystalline materials can be further characterized by their grain structure (eg particle size distributions, shape of the grains, texture, .... etc.).
• an improved bending strength of the ceramic is understood to mean that in the case of a three-point bend, the force leading to breakage is increased.
• the skilled person is for example the
• the sample geometry differs from DIN EN 843-1 (2008) in that the samples have a dimension of 20 ⁇ 40 ⁇ 0.38 mm 3 or 20 ⁇ 40 ⁇ 0.63 mm 3 .
• thermal shock resistance the resistance or the resistance of the copper layer against delamination of the ceramic of a copper-ceramic substrate, wherein the resistance as a result of at least one temperature change of the copper layer relative to the ceramic is determined.
• Improved thermal shock resistance means that the number of resistive temperature changes increases.
• an improved wire bonding means that the force with which the bonding wire can be detached from the copper surface of a copper-ceramic composite is increased.
• Ceramic substrate For example, if an alumina material contains a high proportion of elongated grains, then the mean grain form factor becomes
• Alumina have a relatively low value.
• the mean particle shape factor of Ra is preferably (Al 2 03) of the alumina> 0.60, more preferably> 0.80. Suitable methods by which the shape of the
• Alumina grains for example, already in the production of
• the shape of the Al 2 03 grains by sintering time For example, the shape of the Al 2 03 grains by sintering time and
• Al 2 O 3 substrates with which the above-mentioned form factor Ra (Al 2 O 3 ) can be realized in the final copper-ceramic composite are commercially available or can be obtained by standard methods.
• the particle sizes of the aluminum oxide in the range of 0.01 ⁇ to 25 ⁇ , more preferably in the range of 0.3 ⁇ to 23 ⁇ , more preferably in the range of 0.5 ⁇ to 20 ⁇ .
• these values are not to be considered as strict upper and lower limits for the particle size distribution, but may vary by +/- 10%. In a preferred embodiment, however, it is the lower limit, which is not exceeded, and
• Alumina In this preferred embodiment therefore has the
• Alumina does not have grains lying outside the above ranges. It is therefore b> 0.01 ⁇ and d ma x (Al203) ⁇ 25 ⁇ , more preferably d ma x (Al203) ⁇ 23, more preferably dmin (Al 2 03)> 0.5 microns and a d m X (Al 2 03) ⁇ 20 ⁇ ⁇ where d m in (Al 2 03), and d m a X (Al 2 O 3 ) are the minimum and maximum grain sizes of the alumina.
• Grain sizes of the alumina in these areas may be another
• Improvement of mechanical strength and thermal conductivity can be achieved.
• a ceramic starting material with an appropriate particle size distribution AI2O3- the desired Al can two 03-grit sizes in copper ceramic composite can be adjusted.
• Such ceramics are commercially available or can be obtained by standard methods.
• a fine adjustment of the grain sizes can optionally by a thermal treatment of the
• Grain size distribution with reference to the number of grains i.e.
• Mass distribution or the volume of the grains.
• Ceramic substrate has a number distribution of grain sizes in which at most 5% of the grains have a grain size of less than 0.1 ⁇ m, more preferably less than 0.3 ⁇ m, even more preferably less than 0.5 ⁇ m; and / or in which at least 95% of the grains have a particle size of less than 15 ⁇ m, more preferably less than 10 ⁇ m, even more preferably less than 7 ⁇ m.
• characteristic values of a grain size distribution include its dso value, ds value and d95 value.
• dso value which is often referred to as median value
• 50% of the grains have a diameter smaller than the dso value.
• the arithmetic mean value of a grain size distribution results from the sum of the grain sizes of the individual grains divided by the number of grains.
• the number distribution of the grain sizes of the aluminum oxide preferably has a d95 value of ⁇ 15.0 ⁇ m, more preferably in the range of 4.0 ⁇ m to 15.0 ⁇ m, more preferably in the range of 4.5 ⁇ m to 10.0 ⁇ m, even more preferably in the range of 5.0 ⁇ to 8.0 ⁇ .
• the ds value of the particle size distribution of the aluminum oxide is preferably> 0.1 ⁇ ; More preferably, the ds value is in the range of 0.1 ⁇ to 2.5 ⁇ , more preferably in the range of 0.3 ⁇ to 2.5 ⁇ , even more preferably in the range of 0.5 ⁇ to 2.0 ⁇ .
• Ceramic substrate in the copper-ceramic composite can be achieved.
• a preferred d.sub.50 value of the particle size distribution of the aluminum oxide is, for example, in the range from 1.0 .mu.m to 3.0 .mu.m. In the context of the present invention, it may be preferable for the ds, d 5 and d 5 o values of the particle size distribution of the aluminum oxide to be selected such that they satisfy the following condition:
• the alumina has a number distribution of grain sizes with a median dso and an arithmetic mean darith, the ratio of dso to darith (ie dso / darith) being in the range of 0.75 to 1.10, more preferably in the range of 0.78 to 1.05, more preferably in the range of 0.80 to 1.00. This can be a further optimization of the mechanical strength and
• Thermal conductivity of the ceramic substrate in the metal-ceramic composite can be achieved. Suitable methods by which the symmetry of the particle size distribution in the alumina, for example, already in the preparation of the
• the symmetry of the particle size distribution can be influenced by sintering time and sintering temperature in the production of the starting substrate.
• Symmetry values can be realized, are commercially available or can be obtained via standard methods.
• the width of the grain size distribution can be expressed by the ratio of ds value to d95 value. In a preferred embodiment, the
• Alumina has a number distribution of grain sizes with a ds value and a d95 value, the ratio of ds to d5 being in the range of 0.1 to 0.4, more preferably in the range of 0.11 to 0.35, more preferably in the range of 0.12 to 0.30 is.
• a suitable thickness of the ceramic substrate in a copper-ceramic composite is known to those skilled in the art.
• the ceramic substrate has a thickness of at least 70% of its area, more preferably at least 90% of its area
• a thickness of the ceramic substrate is, for example, about 0.38 mm or about 0.63 mm.
• the thickness of the ceramic substrate (D cer ) and the median dso of the grain size number distribution of the alumina in the ceramic substrate are selected such that the ratio D cer to dso (ie D cer / dso) ranges from 0.001 to 0.01, more preferably Range of 0.002 to 0.009, more preferably in the range of 0.004 to 0.008.
• the thickness D cer of the ceramic substrate is determined at one point and divided by the median dso the particle size distribution of the alumina.
• the ratio D ce r / dso is at least 70%), more preferably at least 90%> of the area of the ceramic substrate in the range of 0.05 to 0.40.
• the alumina may still be reinforced with zirconia (Zr0 2 ).
• Zr0 2 -reinforced A1 2 0 3 contains, based on its total mass, the zirconium oxide usually in a proportion of 0.5-30 wt%.
• the zirconium oxide in turn may optionally be doped with one or more doping oxide (s), in particular yttrium oxide, calcium oxide, cerium oxide or magnesium oxide, usually in an amount of up to 0.01% by weight or even up to 5% by weight, based on the total mass zirconia and alumina.
• doping oxide s
• the ceramic substrate preferably contains at least 65% by weight of A1 2 0 3 . If there is no ZrO 2 for reinforcing the A1 2 0 3 , the ceramic substrate may consist, for example, of at least 95% by weight, more preferably 96% by weight, of A1 2 0 3 .
• the ceramic substrate may be used
• the ceramic substrate may for example have a thermal conductivity of> 20 W / mK, and / or a bending strength of> 400 MPa.
• the ceramic substrate may be in the form of a single substrate.
• the ceramic substrate may have one or more (preferably rectilinear) predetermined breaking lines which divide the ceramic substrate into two or more regions and at least one of these regions Metal coating (eg a coating of copper or a copper alloy) is applied.
• Metal coating eg a coating of copper or a copper alloy
• Suitable dimensions (length x width) of the ceramic substrate (either as a single substrate or as a multiple substrate) in a metal-ceramic composite are known to the person skilled in the art.
• the ceramic substrate may be a
• a coating of copper or a copper alloy is present on the ceramic substrate.
• DCB-method Direct-Copper-Bonding
• AMB-method Active-Metal-Brazing
• the particle sizes of the copper or the copper alloy are in the range of 10 ⁇ to 300 ⁇ , more preferably in the range of 15 ⁇ to 250 ⁇ , more preferably in the range of 20 ⁇ to 210 ⁇ .
• the copper or copper alloy still shows good adhesion to a ceramic substrate even if the copper-ceramic composite is common
• the copper or copper alloy with these grain sizes enables efficient wire bonding.
• a metal coating shows a good wire bonding when the strongest possible connection to a bonding wire is formed, thereby minimizing the risk of unwanted detachment of the bonding wire.
• these values are not to be regarded as strict upper and lower limits for the particle size distribution, but may vary by +/- 10%. In a preferred embodiment, however, it is the lower limit, which is not exceeded, and the upper limit, which is not exceeded, the
• the copper or copper alloy does not have grains that are outside the above ranges. It is thus preferred that dmin (Cu)> 10 ⁇ ⁇ d max (Cu) ⁇ 300 ⁇ , more preferably dmin (Cu)> 15 ⁇ ⁇ dmax (Cu) ⁇ 250 ⁇ , more preferably dmin (Cu)> 20 ⁇ and d max (Cu) ⁇ 210 ⁇ , where d m in (Cu) and d max (Cu) are the minimum and maximum grain sizes of the copper, respectively.
• the desired particle sizes can be set in the copper-ceramic composite become.
• Such copper foils are commercially available or can over
• the copper or copper alloy has a number distribution of grain sizes in which at most 5% of the grains have a particle size of less than 15 ⁇ m, preferably less than 20 ⁇ m, more preferably less than 25 ⁇ m; and / or in the at least 95% of the grains have a particle size of less than 250 ⁇ , preferably less than 230 ⁇ , still
• Copper alloy a d95 value of ⁇ 250 ⁇ , more preferably in the range of 140 ⁇ to 250 ⁇ , more preferably in the range of 140 ⁇ to 230 ⁇ , even more preferably in the range of 150 ⁇ to 200 ⁇ on.
• the ds value of the particle size distribution of the copper or the copper alloy is preferably> 15 ⁇ m;
• the ds value is in the range of 15 ⁇ to 80 ⁇ , more preferably in the range of 20 ⁇ to 75 ⁇ , even more preferably in the range of 25 ⁇ to 70 ⁇ .
• the thermal shock resistance and the bonding behavior of the copper or the copper alloy can be achieved.
• a preferred d.sub.50 value of the particle size distribution of the copper or the copper alloy is, for example, in the range from 55 .mu.m to 115 .mu.m. In the context of the present invention, it may be preferable for the ds, d 5 and dso values of the particle size distribution of the copper or the copper alloy to be selected such that they satisfy the following condition:
• the copper or copper alloy has a number distribution of grain sizes with a median dso and an arithmetic mean d ar ith, where the ratio of dso to d ar ith (ie
• symmetry value S (Cu) of the grain size-number distribution of copper or copper alloy in the range of 0.75 to 1.10, more preferably in the range of 0.78 to 1.05, still more preferably in
• Processing temperature or a rolling process can be influenced.
• Copper starting films with which in the final copper-ceramic composite the above Symmetry values can be realized are commercially available or can be obtained via standard methods.
• the copper or copper alloy has a number distribution of grain sizes with a ds value and a d95 value, the ratio of ds to d5 being in the range of 0.1 to 0.4, more preferably in the range of 0 , 11 to 0.35, more preferably in the range of 0.12 to 0.30.
• shape of an individual grain can be expressed by its form factor R K , which is the ratio of the maximum grain diameter dK, max to the grain diameter dK, ortho, determined to be perpendicular to dK, max
• Suitable methods by which the shape of the grains in the copper, for example, already in the copper starting film can be adjusted, are known in the art.
• the grain shape in a copper foil may be influenced by a suitable processing temperature or a rolling process. Copper starting films, which can be used in the final copper-ceramic composite of the above average grain shape factor Ra (Cu) can be obtained commercially available or can be obtained via standard methods.
• a suitable thickness of the coating of copper or a copper alloy in a copper-ceramic composite is known in the art. As explained below, at some points of the coating, in particular in edge regions, a portion of the copper or the copper alloy can be removed again, for example, to form edge attenuations. Therefore, it is possible within the scope of the present invention that the thickness of the metal coating varies.
• the coating of copper or a copper alloy has a thickness in the range of 0.2-1.2 mm over at least 70% of its area.
• the thickness is about 300 ⁇ .
• the thickness of the coating of copper or a copper alloy (Dcu) and the median value dso of the grain size number distribution of the copper or the copper alloy are selected so that the ratio Dc u to dso is in the range of 0.05 to 0.40.
• the thickness Dc u of the copper or the copper alloy is determined at a point of the coating and divided by the median value dso of the particle size distribution of the copper or the copper alloy.
• the ratio Dcu / dso is at least 70%>, more preferably at least 90%> of the area of the coating of copper or a copper alloy in the range of 0.05 to 0.40.
• the copper of the coating has a purity of> 99.50%, more preferably> 99.90%, more preferably>99.95%> or even>99.99%>.
• the coating of copper or a copper alloy is applied to the ceramic substrate by a DCB method.
• a conventional DCB method can have, for example, the following method steps:
• Heating the composite to a temperature ⁇ 1083 ° C. (for example a temperature in the range of 1065-1080 ° C.),
• spinel crystallites may be present between the coating of copper or a copper alloy and the ceramic substrate (e.g., copper-aluminum spinels).
• the coating of copper or a copper alloy may be mounted on only one side of the ceramic substrate.
• both sides (i.e., top and bottom) of the ceramic substrate are provided with the coating of copper or a copper alloy.
• An exemplary copper-ceramic composite in which a ceramic substrate 1 has a coating 2 made of copper or a copper alloy both on its lower side and on its upper side is shown in FIG.
• Coating 2 are made of copper or copper alloy is shown in FIG. As will be explained below, the individual metallized regions can be separated from one another by predetermined breaking lines (not shown in FIG. 3), so that a separation of these regions can take place by breaking along these predetermined breaking lines.
• the coating of copper or a copper alloy may at least partially have a structuring.
• Structuring of the metal coating can be carried out in a known manner, in particular via an etching process (for example using a
• Etching mask In the etching process, the copper or the copper alloy can be completely removed in partial areas, so that in these partial areas the surface of the ceramic substrate is exposed. Furthermore, it is also possible that the
• Coating of copper or a copper alloy one or more depressions (preferably round recesses), for example, in the etching process be obtained, for example, that only partially removed the copper or copper alloy in the region of the mounting recess and the
• Recesses through the copper or copper refining through to the ceramic surface are provided.
• Further improvement in the properties of the copper-ceramic composite can be realized by matching the grain properties of the copper or copper alloy and the grain properties of the aluminum oxide in the ceramic substrate.
• the ratio of dso (Al 2 O 3 ) to dso (Cu) is in the range of 0.008 to 0.055, more preferably in the range of 0.010 to 0.045.
• the copper or copper alloy has a number distribution of grain sizes with a median dso, one
• Arithmetic mean d ar ith and a symmetry value S (Cu) dso / d ar ith, the alumina has a number distribution of grain sizes with a median value dso, an arithmetic mean darith and a symmetry value
• S (A1 2 03) dso / darith, where S (A1 2 0 3 ) and S (Cu) satisfy the following condition: 0.7 ⁇ S (A1 2 0 3 ) / S (Cu) ⁇ 1, 4 , More preferably, S (A1 2 0 3 ) and S (Cu) satisfy the following condition:
• the average particle shape factor of the alumina Ra (Al 2 0 3) and the mean particle shape factor of the copper or copper alloy Ra satisfy (Cu) the following condition: 0.5 ⁇ Ra (Al 2 O 3) / Ra (Cu) ⁇ 2.0.
• the alumina has particle sizes in the range of d m in (Al 2 0 3 ) to d m ax (Al 2 0 3 ), the copper or copper alloy has grain sizes ranging from d m in (Cu) to d max (Cu),
• the present invention relates to a module comprising at least one metal-ceramic composite according to the above description and one or more bonding wires.
• the bonding wire or wires are bonded to the copper or copper alloy coating.
• the module may also include one or more electronic components, such as one or more chips.
• the grain structures of the aluminum oxide of the ceramic substrate and the copper or copper alloy of the metal coating are determined in the context of the present invention as follows: Grain size distribution of the aluminum oxide of the ceramic substrate
• SEM image From the surface of the ceramic substrate, a scanning electron micrograph (SEM image) is taken. A special sample preparation in the form of a microsection is not required. The SEM photograph is taken at a location of the ceramic substrate that was previously copper-plated and exposed by etching.
• the grain sizes are determined by a line-cutting method.
• the magnification is chosen so that the line grid cuts at least 50 A Ch grains. If the ceramic substrate still contains grains of other chemical composition, for example ZrC grains, these can be distinguished well in the SEM image by means of secondary electron contrast from the AbOs grains and are thus not included in the subsequent calculations.
• two parallel lines in the x-direction and two parallel lines in the y-direction were respectively placed in the optical micrograph.
• the lines divide the recording into three equally wide stripes. This is shown schematically in FIG.
• this length L is taken as a grain size.
• a grain size is obtained.
• two values for a grain both of which are included in the determination of the particle size distribution.
• Grain size distribution from which in turn the d 5 , dso and d95 values as well as the arimethical mean value darith can be determined.
• the dso value which is frequently also referred to as median value
• 50% of the grains have a diameter which is smaller than the dso value.
• 5% of the grains have a diameter smaller than this ds value
• 95% of the grains have a diameter smaller than this d95 value.
• the arithmetic mean of the particle size distribution results from the sum of the grain sizes of the individual grains divided by the number of cut grains. Grain size distribution of copper or copper alloy
• the grain sizes are determined by a line-cutting method.
• the magnification is chosen so that the line grid cuts at least 50 grains.
• Grain sizes of the AI2O3 are therefore determined in a plane which is parallel to the coated substrate surface or coplanar with it.
• Form factor of individual grains mean grain form factor Aluminum oxide
• SEM image used was also used in the determination of the particle size distribution.
• VON dK, ortho TO dK, max, i. RK dK, ortho / dK, max.
• FIG. 3 This is shown schematically in FIG. 3 on the basis of a grain having an elliptical grain structure. The more the shape of a grain in its two-dimensional
• Circularity / roundness of the grains is determined for at least 50 grains of the SEM image. Usually, the grains are evaluated, which were also cut in line intersection of the lines.
• the mean grain shape factor of the alumina then results from the arithmetic mean of the shape factors of the individual AhOs grains (ie sum of individual shape factors divided by the number of grains examined).
• a typical process process which is preferably used in the context of the present invention for applying the copper coating to the ceramic substrate is known, for example, from the publications US Pat. No. 3,744,120, US Pat. No. 3,994,430, EP 0 085 914 A or DE 23 19 854 A, the corresponding disclosure is incorporated by reference into the present invention.
• Copper oxide layer results.
• the resulting copper foil is then positioned on a ceramic substrate and the ceramic substrate / copper foil composite is heated to a bonding temperature between about 1025-1083 ° C, thereby forming a metallized ceramic substrate.
• the copper foil thus forms a coating after bonding.
• the resulting metallized ceramic substrate is cooled.
• the bonding of ceramic substrate and copper foil is carried out in an oven, so-called bonding furnaces generally being used.
• tunnel ovens include, inter alia, an elongated tunnel-like furnace space (also called muffle) and a transport device with a transport element, for example in the form of a flexible and heat-resistant conveyor belt for transporting the
• the Ceramic substrates are positioned together with the copper foil on a carrier on the conveyor belt and then passed through in the bonding oven by the conveyor belt driven a heating area in which the required
• Bonding temperature is reached.
• the resulting ceramic substrate / copper foil composite is recooled according to the invention.
• This method can be used in principle for the production of single-sidedly metallized ceramic substrates as well as for the production of double-sided metallized substrates.
• the production of double-sided metallized substrates usually takes place via a two-stage bonding process, i. using a two-stage single-layer process (SLB method).
• a two-stage bonding process is preferably used. In this two-stage bonding process for the production of double-sided
• the ceramic is connected in two furnace passes with the copper foils on the opposite sides of the ceramic substrate.
• a ceramic substrate is first positioned on a support and then placed on the overhead, i. on the side facing away from the carrier, covered with a copper foil. By heat, this side of the ceramic substrate is connected to the metal layer and then the
• the substrate is turned over and in a second bonding step, the other side of the substrate is coated in the same way with a metal layer, i. the copper foil, provided.
• the following examples show how the shape factor of the AhOs grains of the ceramic substrate affects the mechanical strength of a copper-ceramic composite.
• both the top and the bottom of the ceramic substrate was provided with a copper coating.
• the Copper coating was first bonded to one side of the ceramic substrate by means of the SLB process. Subsequently, the opposite side of the ceramic substrate by means of the SLB method with another
• Copper coating so that a copper-ceramic substrate is formed in which a copper foil is bonded on both sides of the ceramic.
• One of the two copper coatings was subsequently patterned in each sample via an etching process (same structuring for all samples).
• the substrates consisted of 96% by weight of Al 2 O 3, but differed in their grain structure.
• the ceramic substrate had the following dimensions:
• Thickness of the ceramic substrate 0.38 mm
• Length x width of the ceramic substrate 190 x 140 mm 2
• the copper coating each had a thickness of 0.3 mm.
• FIG. 5 shows a SEM image of the surface of the ceramic substrate of K-K-V 1, by means of which the grain structure of the Al 2 O 3 was determined.
• the mean shape factors Ra (Al 2 O 3 ) of the AI 2 O 3 of the samples KKV 1 to KKV 3 are listed in Table 1.
• the flexural strength of the ceramic substrate in the copper-ceramic composite was determined.
• the breaking force was determined by means of a three-point bend.
• the measurement was based on DIN EN 843-1 (2008), whereby the sample geometry deviated from DIN EN 843-1 (2008) in that the samples had a dimension of 20 x 40 x 0.38 mm 3 or 20 x 40 x 0.63 mm 3 .
• AI2O3 grain structure according to the invention which has a high proportion of relatively round or circular grains, to achieve an improvement in the mechanical strength.

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